Giardia lamblia, a pathogenic protozoan, is the most frequently isolated enteric parasite in the United States, and is an important cause of debilitating diarrhea throughout the world. Accumulating information about giardiasis suggests that the host immune response plays an important role in protection from disease, in clearance of the parasite, and in certain instances, in production of disease. It is the purpose of this proposal to study the host response, specifically to define and examine the gastrointestinal immune events which occur during infection. The murine model of Giardia muris infection will be used; this model is well established and has many similar characteristics to human giardiasis. Intestinal lymphoid cells from Peyer's patches, epithelium, lamina propria, and mesenteric lymph nodes will be isolated at specific time points during the course of Giardia muris infection and then characterized with monoclonal antibodies. The cytotoxic capability of certain populations of these cells against Giardia trophozoites will then be examined. The intestinal epithelium is rich in natural killer cells, therefore, natural killer cell cytotoxicity will first be studied. Antibody dependent and mitogen-induced (phytohemagglutinin) cellular cytotoxicity then will be examined by using combined intraepithelial and lamina propria lymphocyte populations. Intestinal macrophages will be isolated and examined for their ability to ingest and kill Giardia trophozoites, and for their ability to process and present antigen to lymphocytes for antigen-specific lymphocyte transformation. Humoral responses will be studied by following the development of gastrointestinal and systemic antibody and comparing its course to cellular changes. The ability of mouse serum to kill Giardia and act as an opsonin will also be examined. Peyer's patch, mesenteric lymph node and lamina propria cells will be studied for their ability to produce anti-Giardia specific antibody by using a plaque forming cell assay. The appearance of anti-Giardia specific plasma cells in gut mucosa will be studied by immunoperoxidase staining. Finally, the development of these cellular and humoral events will be correlated with the time course of Giardia antigen appearance in Peyer's patches and mesenteric lymph nodes. It is expected that the results of these studies will add to an understanding of gastrointestinal immune defense mechanisms and lead to an effective immunoprophylactic strategy in giardiasis.